1. Field of the Invention
The present invention relates to an energy discharge apparatus for a coil in an electric machine, and in particular in a brushless generator. The energy discharge apparatus is used to dissipate a quantity of stored magnetic energy in the coil.
2. Description of the Related Art
It is well known in the art that generator load-dump response time is often dominated by the field winding decay time of the quantity of stored magnetic energy. The prior art has made use of various schemes in brushed generators to speed the decay of the quantity of the stored magnetic energy in a field winding.
For instance, U.S. Pat. No. 6,191,562 by Mueller et al. makes use of a circuit configuration in a claw-pole generator having a field winding and a generator winding. The circuit configuration is for the degradation of the stored magnetic energy of a main field winding. The circuit configuration includes a power switch, a clock control and a zener diode. The circuit configuration is incorporated with other electronics for control of the field winding. In a brushless generator environment the use of direct electrical connections to the main field winding is not available however.
U.S. Pat. No. 5,023,539 by Miller et al. provides a method and apparatus for controlling field current in an alternator. The apparatus includes a regulator, a first insulated gate transistor controlled by a first optocoupler and a first operational amplifer, a second insulated gate transistor controlled by a second optocoupler and a second operational amplifier. The first transistor is in series with the field winding and provides a pulse width modulated signal to the field winding. The second transistor is in parallel with the field winding and serves to auto-commute the current through the field winding to maintain a desired regulated battery voltage. This apparatus has the disadvantage of complexity in a brushless generator application. For reliability it is undesirable to incorporate complex electronics on a rotor of the brushless generator. Furthermore, as stated above, direct electrical connections to the main field winding are not available between a stator and the rotor. Furthermore, a rate of decay of the energy in the field winding is limited by an RL network.
In a brushless generator application the rate of decay of energy in the field winding is usually limited largely by the RL network formed by the winding itself and the total resistance of the current path.
What is needed is a novel apparatus for a rapid degradation of the quantity of stored magnetic energy and which can be directly connected to the field winding of a brushless generator.